Computer and Internet Security

Introduction

• Both individuals and companies are vulnerable to data theft and hacker attacks that can compromise data, corrupt files and crash systems.

• For these reasons, security is crucial to the success of e-business and Web services.

Introduction

• Five fundamental requirements for a successful, secure transaction: – Privacy

• How do you ensure that the information you transmit over the Internet has not been captured or passed to a third party without your knowledge?

– Integrity• How do you ensure that the information you send or receive

has not been compromised or altered? – Authentication

• How do the sender and receiver of a message verify their identities to each other?

Introduction

– Authorization• How do you manage access to protected resources on

the basis of user credentials?

– Non-repudiation. • How do you legally prove that a message was sent or

received?

Ancient Ciphers to Modern Cryptosystems

• Cryptography – Transforms data using a cipher

• Cryptosystem– A mathematical algorithm for encrypting messages

• A key– A string of digits that acts as a password– Input to the cipher

• Unencrypted data is called plaintext; encrypted data is called ciphertext.

Ancient Ciphers• The two main types of ciphers were

– substitution ciphers (Caesar Cipher)• “abc” “def”

– transposition ciphers• “security” “scrt euiy”

• Complicated ciphers combination of substitution ciphers transposition ciphers.– E.g., using the substitution cipher in transposition cipher, the

word "security" would encrypt to "tdsu fvjz”.

• Problem of many historical ciphers – their security relied on the sender and receiver to remember

the encryption algorithm and keep it secret.

Modern Cryptosystems

• Modern cryptosystems are digital. • Their algorithms are based on the individual

bits or blocks (a group of bits) of a message, rather than letters of alphabet.

• Encryption and decryption keys are binary strings with a given key length.– For example, 128-bit encryption systems have a

key length of 128 bits.

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The language of cryptography

symmetric key crypto: sender, receiver keys identicalpublic-key crypto: encryption key public, decryption key secret

(private)

plaintext plaintextciphertext

KA

encryptionalgorithm

decryption algorithm

Alice’s encryptionkey

Bob’s decryptionkey

KB

Secret-Key Cryptography

• In the past, organizations wishing to maintain a secure computing symmetric cryptography, also known as secret-key cryptography.– Uses the same secret key to encrypt and decrypt a

message

secret-key cryptography

Secret-Key Cryptography

• Have to find a secure way to exchange the secret key before communications– What is the best method? (use FedEx? What are

the problems for it?)• Not efficient• Not secure• Authentication problem• Etc.

Secret-Key Cryptography– Another way

• Have a central authority - an key distribution center (KDC)

Secret-Key Cryptography - Data Encryption Standard (DES)

• One of the most commonly used symmetric encryption algorithms

• Using block cipher– DES has a key length of 56 bits and encrypts data

in 64-bit blocks.

Public-Key Cryptography

• Developed by Whitfield Diffie and Martin Hellman in 1976 at Stanford University.

• Two keys – a public key • Freely distributed to outside world

– a private key• Kept secret by its owner

Public-Key Cryptography

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Public key cryptography

plaintextmessage, m

ciphertextencryptionalgorithm

decryption algorithm

Bob’s public key

plaintextmessageK (m)B

+

K B+

Bob’s privatekey K B

-

m = K (K (m))B+

B-

RSA Algorithm

• RSA – the most commonly used public-key algorithm

• Developed by MIT professors Ron Rivest, Adi Shamir, and Leonard Adleman in 1977

• Play an important and crucial role in e-Commerce transactions

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Public key encryption algorithms

need K ( ) and K ( ) such thatB B. .

given public key K , it should be impossible to compute private key K B

B

Requirements:

1

2

RSA: Rivest, Shamir, Adelson algorithm

+ -

K (K (m)) = m BB- +

+

-

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RSA: Choosing keys1. Choose two large prime numbers p, q. (e.g., 1024 bits each)2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”).4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ).

5. Public key is (n,e). Private key is (n,d).

K B+ K B

-

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RSA: Encryption, decryption0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, computec = m mod

ne (i.e., remainder when m is divided by n)e

2. To decrypt received bit pattern, c, computem = c mod

nd (i.e., remainder when c is divided by n)d

m = (m mod n)e mod

ndMagic

happens!c

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RSA: another important property

The following property will be very useful later:

K (K (m)) = m BB- + K (K (m)) BB

+ -=

use public key first, followed

by private key

use private key first,

followed by public key

Result is the same!

Authentication using public-key algorithm

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Authentication: yet another tryGoal: avoid playback attack

Failures, drawbacks?

Nonce: number (R) used only once –in-a-lifetimeap4.0: to prove Alice “live”, Bob sends Alice nonce, R.

Alicemust return R, encrypted with shared secret key

“I am Alice”

RK (R)A-B

Alice is live, and only Alice knows key to encrypt

nonce, so it must be Alice!

Key Agreement Protocols

• A drawback of public-key algorithm – not efficient for sending large amounts of data.

• Public key algorithms are not used to replace secret-key algorithm

• Use Public key algorithms to exchange secret keys securely.

• Key Agreement Protocol– The process by which two parties can exchange

keys over unsecure medium

Digital Envelope• The most common key agreement protocol

Key Management

• Maintaining the secrecy of private keys is crucial for keeping cryptographic systems secure.

• Most compromises in security result from poor key management

• Key generation– The process by which keys are created

Key Management

• Brute-force cracking– Try to decrypt a message by using every possible

decryption key• If the subset is too small for key generation,

then the encrypted data is more susceptible to brute-force attacks

Have a key-generation program that can generate a large number of keys as randomly as possible

Digital Signatures

• The electronic equivalents of written signatures• Developed to be used in public-key cryptography

to solve the problems of authentication and integrity

• To create a digital signature, a sender first takes the original plaintext message and runs it through a hash function, which is a mathematical calculation that gives the message a hash value

Digital Signatures

• A hash value identifies a message uniquely.– The hash value is also known as a message digest

• Message integrity– If the hash value of the original message matches

the message digest included in the signature– The message has not been altered in transmission.

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Digital Signatures

Cryptographic technique analogous to hand-written signatures.

• sender (Bob) digitally signs document, establishing he is document owner/creator.

• verifiable, nonforgetable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

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Digital Signatures

Simple digital signature for message m:• Bob signs m by encrypting with his private key KB,

creating “signed” message, KB(m) --

Dear AliceOh, how I have missed you. I think of you all the time! …(blah blah blah)

Bob

Bob’s message, m

Public keyencryptionalgorithm

Bob’s privatekey K B

-

Bob’s message, m, signed

(encrypted) with his private key

K B-(m)

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Digital Signatures (more)• Suppose Alice receives msg m, digital signature KB(m)

• Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m.

• If KB(KB(m) ) = m, whoever signed m must have used Bob’s private key.

+ +-

-

- -+

Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m’.

Non-repudiation: Alice can take m, and signature KB(m) to court and prove that

Bob signed m. -

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Message Digests

Computationally expensive to public-key-encrypt long messages

Goal: fixed-length, easy- to-compute digital “fingerprint”

• apply hash function H to m, get fixed size message digest, H(m).

Hash function properties:• many-to-1• produces fixed-size msg

digest (fingerprint)• given message digest x,

computationally infeasible to find m such that x = H(m)

large message

mH: HashFunction

H(m)

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large message

mH: Hashfunction H(m)

digitalsignature(encrypt)

Bob’s private

key K B-

+

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

KB(H(m))-encrypted msg digest

KB(H(m))-encrypted msg digest

large message

m

H: Hashfunction

H(m)

digitalsignature(decrypt)

H(m)

Bob’s public

key K B+

equal ?

Digital signature = signed message digest

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Certification Authorities

• Certification authority (CA): binds public key to particular entity, E.

• E (person, router) registers its public key with CA.– E provides “proof of identity” to CA. – CA creates certificate binding E to its public key.– certificate containing E’s public key digitally signed by CA – CA says

“this is E’s public key”

Bob’s public

key K B+

Bob’s identifying informatio

n

digitalsignature(encrypt)

CA private

key K CA-

K B+

certificate for Bob’s public

key, signed by CA

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Certification Authorities• When Alice wants Bob’s public key:

– gets Bob’s certificate (Bob or elsewhere).– apply CA’s public key to Bob’s certificate, get

Bob’s public key

Bob’s public

key K B+

digitalsignature(decrypt)

CA public

key K CA+

K B+

Secure Sockets Layer (SSL)

• Currently, most e-businesses use SSL for secure online transactions

• The Secure Sockets Layer (SSL) protocol is a non-proprietary protocol commonly used to secure communication betweens two computers on the Internet and the Web.

• Use TCP/IP in the sockets

Secure Sockets Layer (SSL)

• In a transaction using SSL, the sockets are secured using public-key cryptography.

• SSL implements public-key cryptography using the RSA algorithm and digital certificates to authenticate the server in a transaction and to protect private information as it passes over the Internet.

Secure Sockets Layer (SSL)

Client Server

Socket Socket

1. Encrypt packets with server’s public key

2. Send packets to the server

1. Receive packets from the client

2. Decrypt packets with server’s private key

Security Attacks

• Denial-of-Service (DoS) Attacks• Viruses and Worms• Eavesdrop: intercept messages• Actively insert messages into connection• Impersonation: can fake (spoof) source address in

packet (or any field in packet)• Hijacking: “take over” ongoing connection by

removing sender or receiver, inserting himself in place

Network Security

• Firewalls• Intrusion Detection Systems• Steganography– Hiding information within other information